The present invention relates to a process and device, herein a method and apparatus with which to determine the volumes of liquid droplets, in particular to determine the quantities of precipitates.
High requirements on measurement techniques are placed on reliable determination of volumes of minimal liquid quantities in droplet form. The measurement range is approximately 1 xcexcltr to the maximum drop size. It is known to determine volume by weighing, but this procedure requires scales for comparatively small forces which are susceptible to such interference as air flows, temperature changes, microphonics etc. Using weighing, it is practically impossible to measure the particular volume of each droplet when such droplets follow each other rapidly.
It is further known to ascertain the droplet volume optically by analyzing the image of the droplet shape by resort to their shadows or using similar procedures. In such tests, optical transparency (particle-induced changes in transmission) and reflectivity of the liquid droplet negatively affect measurement accuracy.
Apparatus to measure the boundary-layer tension between two liquids of different densities is known from U.S. Pat. Nos. 4,697,451 and 4,569,226. The two liquids are present inside a cylinder where they form a boundary layer. Using a pipette, one liquid is dripped into the other underneath latter""s liquid level. The pipette is made of an electrically conducting material and is connected to a voltage source. The liquid in the lower part of the cylinder is connected to an electrometer time-integrating the charge of the transiting liquid droplets. Furthermore the drop frequency is transmitted through the electrometer. Using a variable voltage source, the boundary-layer capacitance of the droplets formed in the liquid is indirectly determined iteratively from the slope of a straight line.
The European patent document A 0,286,419 describes apparatus with which to measure the pH value of a solution. Droplets are formed at the end of fiber optics, the end of the fiber optics being located between the plates of a capacitor.
The German patent document A 26 53 166 discloses apparatus with which to measure the concentration of components in a flowing liquid. This apparatus makes use of the electrokinetic effect. The liquid is converted into a stream of droplets transporting the generated stream drained by a drain electrode of a collecting vessel. Volumetry of the droplets is not provided.
The objective of the present invention is to create such a method and apparatus that liquid-droplet volumes can be determined more accurately and more economically.
This problem is solved by the falowing and apparatus.
Inventive method for measuring the volumes of liquid droplets is characterized by the following method steps:
separating a liquid volume into individual droplets,
electrically charging the droplets relative to their ambience, such a charge being a function of the drop""s potential difference and its capacitance relative to its ambience,
measuring the droplet""s capacitance or a value dependent on the capacitance, and
determining the droplet volume from the measured potential jump or from a derivative value.
Inventive method for measuring liquid drops is characterized by the following method steps:
separating a liquid volume into individual droplets,
raising the droplets to a potential different from that of their ambience,
determining the droplet charges by converting the charges into voltage jumps or a test value dependent on the voltage jumps,
determining the droplet capacitances from the measured voltage jump or from the test value dependent on the voltage jump, and
computing droplet volumes from their capacitances.
Inventive apparatus with which to carry out the two (2) inventive methods set forth immediately above, and characterized by the following elements:
a liquid drop collecting vessel (2, 40) comprising a drop dripper (4, 42) to deliver individual drops,
a device to electrically charge the individual drops formed by the dripper (4, 42),
a measurement device (8, 44) mounted underneath the liquid-droplet collecting vessel (2, 42) connected to a droplet collector electrode (16, 46) to determine the capacitances of the droplets or derivative test values, and
an analyzer to determine the drop volume from the measured capacitance or a derivative test value.
An inventive apparatus as set forth above, further characterized in that the droplet charging device comprises a voltage source (12, 62) connected to the liquid-drop collecting vessel (2) or to the dripper (4) or to the collector electrode (46) to charge them to an electric potential different from that of the ambience.
Inventive apparatus as set forth above further characterized in that the dripper (4) is fitted with a potential guide needle (10).
Inventive apparatus as set forth above further characterized in that the potential guide needle (10) is conductively connected to the liquid droplet trap (2), the drop being connected by this potential guide needle to the potential of the collecting vessel (2).
Inventive apparatus as set forth above further characterized in that the droplet collector electrode (16, 46) is connected to a measurement amplifier of which the output is a peak voltage value which is a function of the voltage jump caused by the change in potential at the collector electrode (16) by the droplet charge, or is proportional to the voltage jump, or is a function of the voltage jump caused by the change in capacitance of the collector electrode (46) due to the droplet capacitance or is proportional to this voltage jump.
Inventive apparatus as set forth above further characterized in that the measurement amplifier is a charge/voltage converter (20, 48).
Inventive apparatus as set forth above further characterized in that a peak voltage detector (24, 50) is connected to the measurement amplifier or charge/voltage converter (20, 48) for interim storage of the peak voltage value until ensuing analysis.
Inventive apparatus as set forth above further characterized in that it includes an exponentiator (26, 52) to raise the peak voltage value to the third power in order to provide a measurement value which is linear with the droplet volume.
Inventive apparatus as set forth above further characterized in that the peak voltage value detector (24, 50) is resettable following retrieval of the peak voltage value.
Inventive apparatus as set forth above further characterized in that the measurement device (8) includes a screening field electrode (14) enclosing the droplet""s falling trajectory.
Inventive apparatus as claimed set forth above further characterized in that it comprises a compensation electrode (22) associated with the collector electrode (16) and together with the collector electrode (16) forming a capacitor of which the capacitance depends on the dielectric constants of the ambient medium and which thereby compensates capacitance changes in drops of equal sizes due to a changing dielectric constant.
Inventive apparatus as set forth above further characterized in that the voltage source (12, 62) comprises a high voltage converter.
Inventive apparatus as set forth above further characterized in that the collector electrode (46) is connected to a constant current source or, through a high resistance, to a voltage source (62).
Inventive apparatus as set forth above further characterized in that a comparator (30,56) is connected to the output of the peak value detector (24, 50) and will emit a transfer command signal in the presence of measurement values and in that the peak value detector is resettable by a one-shot multivibrator (32, 66) driven by a transfer confirmation signal.
The method of the invention makes use of the feasibility to impart charges (relative to their environment) to masses suspended in an insulated manner or floating or falling, on account of the capacities of such masses, such charges being retained for some time even without connection to a voltage source. A liquid, for example a water droplet, is charged by being made to contact an electrode at an electrical potential different from its environment the environment is, for instance, a metal plate at a potential Uce 2 which the droplet then also assumes. The droplet capacitance is determined by the diameter, ie the mass of the droplet and in turn determines the charge accepted by this droplet.
2ce=charging electrode 
A charged and falling droplet retains this charge for some time and shares its charge with the capacitance of a detector electrode. The droplet""s charge generates a voltage jump relative to the environment at the summed capacitance of droplet and detector electrode.
An uncharged and falling droplet increases the capacitance of an electrode detector at an electric potential different from its environment""s according to its own capacitance, thereby generating a potential jump.
The voltage jump is measured and analyzed. Illustratively it will be amplified by a suitable measurement amplifier and will be stored in impedance-converted form in a peak-value detector, whereafter it can be analyzed at a convenient time. After considering the cubic relation between radius and volume of a sphere, a test value is obtained which is directly proportional to volume, assuming of course that the droplet is approximately spherical.
The capacitance C of a sphere of radius R in space is given by C=4xcfx80xcex50(r)R, where xcex50(r) is the electric field permittivity constant.
Together with the (imparted) potential difference, the capacitance C determines the electric charge Q of the droplet according to the expression
Droplet charge Q=CdUce
where Cd is the droplet capacitance.
The droplet charge relative to its environment illustratively can be measured, using a charge/voltage converter, after the droplet makes contact with a detector or collector electrode. For instance the voltage that upon droplet impact on the detector or collector electrode and that will appear at the capacitance of this electrode and at the input of an operational amplifier and will decay according to the time constant of the input impedance, can be measured as a peak value:
Measured peak voltage U=[Cd/(Cme+Cd)]Uce.3
3 me=measurement electrode 
The volume or the mass of the droplet can be calculated from the value of the measured charge at constant known potential difference Uce of the charging electrode. Because of the cubic relation between radius and sphere volume, the potential values must be to the third power to lead to a test value which is linear with volume.
The invention is elucidated below in relation to the drawing showing illustrative embodiments.